With development of industries, an amount of use of papers including public relation medium as well as general documents tends to be drastically increased. Therefore, forests are destroyed as the supply of pulp which is raw material for the paper is increased and the problem due to global warming becomes serious.
An organic dye for showing the color of ink consists of a chromophore and an auxochrome. In general, examples for the chromophore include an azo group (—N═N—), a carbonyl group (>C═O), a nitro group (—NO2), a thiocarbonyl group (>C═S), a nitroso group (—N═O), an ethylene linkage (—C═C—) and a cyano group ((—C═N—), and examples for the auxochrome include an amino group (—NH2), a secondary amino group (—NHR), a tertiary amino group (—NR2), a hydroxyl group (—OH), a sulfone group (SO3), a carboxyl group (—COOH), a mercapto group (—SH), an alkoxy group (—OR) and a halide (—X). The auxochrome improves color strength and dyeability. This auxochrome also improves reactivity since it has a non-bonding electron.
One of a material which readily reacts with the dye and thus decolors the dye is a molecular reaction mechanism in which a bleaching agent (ion) freed from an acid reacts with the dye, and the other is an active oxygen mechanism in which a hydroxy radical (.OH), a peroxy radical (.OOH) or a superoxide radical (.O2—) reacts with the dye.
However, the above compounds oxidate a recording medium and decolor the color of the recording medium or damage the recording medium.
A more stable reaction mechanism which decolors the dye is a method using an organic compound which readily forms a radical by heat or light. These react with other organic compound to form high molecular material or react with the auxochrome of the dye to destroy the chromophore, thereby decoloring the dye. This reaction is a reaction mechanism named as a Dulux process, in which the dye is destroyed by readily reacting with a material readily generating the radical by the heat or light (Heinrich Zollinger, COLOR CHEMISTRY pp. 288-303, VCH (1971)).
Also, a rewritable marking material is used which is referred to as a Leuco dye and can repeatedly alternate between colored and decolored states as its molecular structure is transformed by the heat or light. The rewritable marking material refers a marking material in which an image is formed when energy is given, the image is maintained as it is when the energy is not given any more and the image is disappeared to be reusable when other energy is given. There are various rewritable marking materials and a thermally rewritable marking material is largely used.
This thermally rewritable marking material includes a high molecular/low molecular composite type, a high molecular type and a Leuco dye type. The Leuco dye type is one using color tone change, in which the Leuco dye of two-component dye systems is employed as a pigment and reaction of the dye with a decolorant is used. The Leuco dye which is generally used as a thermal transfer ink is widely used for the purpose of recording, and this thermally rewritable marking material is widely used as it has excellent resolution and stability. However, the thermally rewritable marking material has insufficient repetitivity and is gradually decolored by the heat or light. Therefore, the thermally rewritable marking material is not suitable for long-term storage documents.
In a more developed method for decoloring the dye, the dye is contained in a clathrate compound such as dextrin and a photo decolorant is chemically coupled to the outside thereof, which is used as an ink for toner of a laser beam printer. The aforementioned method has been continuously studied by R. S. Nohr team of Kimberly-Clark (U.S. Pat. No. 5,643,701, U.S. Pat. No. 5,683,843, U.S. Pat. No. 5,721,287, U.S. Pat. No. 6,017,471, U.S. Pat. No. 6,017,661, U.S. Pat. No. 6,017,661, U.S. Pat. No. 6,033,465, U.S. Pat. No. 6,060,200, U.S. Pat. No. 6,120,949, U.S. Pat. No. 6,127,073, U.S. Pat. No. 6,235,095 and U.S. Pat. No. 6,342,305).
Also, studies for the Leuco dye of the same structure are disclosed in U.S. Pat. No. 6,017,386, U.S. Pat. No. 5,922,115, U.S. Pat. No. 6,063,551 and U.S. Pat. No. 6,790,809.
The aforementioned method has a disadvantage that the color is gradually discolored in long-term storage as described in U.S. Pat. No. 6,342,305 or lately suggested method. Also, the method is not able to be applied to liquid inks such as a printing ink, a ribbon ink, a Deskjet ink, inks for various writing instruments, other than the case that the ink is present in solid state such as the toner of the laser beam printer, since the inks are decolored during storage or use thereof as the dye and the decolorant are dissolved in solvent to react with each other.
In addition, although the clathrate compound is not dissolved in the solvent, the clathrate compound may be broken by friction with the recording medium upon recording. In the case of the ribbon ink, since the ink dyed in the ribbon is printed by a dot impact manner, the clathrate compound is broken to cause discoloration of the ribbon.
Particularly, as disclosed by the present applicant in Korean Patent No. 0536056, parking tickets or highway tickets may be exposed at a high temperature of 70 to 100° C. and sunlight and information printed thereon may thus be erased under sever condition. Therefore, although it is theoretically possible to prepare an ink for general purpose other than special purpose using the dye and the decolorant together, there are actually many problems.
Another method (JP1997-286979) has been suggested to provide a decoloring composition, in which an ink is prepared as a mixture with a photo-decolorant and an accelerator and a printed ink can be erased by irradiation of light. Although the patent insists that its ink is decolored only when ultra violet ray (UV) of a predetermined wavelength is irradiated, the patent does not give a specific description how the ink is not discolored by a photo-radical former when exposed to the sunlight during use. Therefore, the patent cannot give the scientific basis how the ink is not discolored during storage and use. Particularly, in a case that the recording medium is a paper, the dye component and hydroxyl group (—OH) in cellulose which constitutes the paper are chemically bonded with each other. Therefore, the ink is not readily erased and an afterimage may be remained on the paper.
Accordingly, although it is theoretically possible to prepare an ink for general purpose other than special purpose using the dye and the decolorant together, there are actually many problems.
Meanwhile, the paper is recycled by removing the ink printed thereon and then remanufacturing it as a recycled paper in a paper mill. However, there are disadvantages that the recycled paper has degraded quality and complex recycling process is required.
Therefore, so far, not only a general-purpose paper but also paper and recording medium including various pass cards which are specially printed and have high preparation cost are discarded after a single use. However, according to an embodiment of the present invention, it is possible to use the recording medium several times or semi-permanently and thus an enormous economic effect can be expected.
Particularly, confidential papers which require to be kept in secret are generally shred or burned up. However, according to the present invention, the content recorded on the recording medium is readily erased and the recording medium can be reused since the printed ink itself is chemically decomposed. Particularly, a pass card or identification card having a magnetic stripe attached thereon is an expensive printed matter which is used in a large amount. However, it has to be burned up so far as it cannot be recycled and causes an environmental problem as the magnetic stripe is not decomposed after discarded. Therefore, repeated use of the paper with the magnetic stripe can largely reduce the environmental problem. Also, a large import substitution effect can be expected by the repeated use of the paper with the magnetic stripe since the material for the magnetic stripe is all imported.